Harper. — Cell-Division in Sporangia and Asci. 499 
plasmic cleavage-segments. The spore-plasma is thus roughly 
marked out into irregular blocks apparently without reference 
to their size or the number of nuclei they contain (Figs. 14-15). 
Further progress of this furrowing cuts the first-formed blocks 
into oblong rounded sausage-shaped masses generally con- 
taining two to four nuclei in a row (Fig. 16). These oblong 
bodies are now divided transversely to form rounded or 
spherical masses each with one or few nuclei (Fig. 17). With 
this the primary cleavage phenomena are complete. The 
multinucleated spore-plasma has been cut up into units with 
one or few nuclei, corresponding to the protospores produced 
by the cleavage in the sporangium of Synchitrium. 
But these are by no means the definitive spores of Pilobolus . 
A further period of growth and cleavage is to be passed through 
before the spores with protective walls are formed ready 
for dissemination. These uninucleated masses are perhaps 
morphologically the equivalents of the Saprolegnia swarm- 
spores, but they have no cell-walls, and are but a transitory 
developmental stage in the Pilobolus sporangium. The 
succeeding periods are, however, at once distinguished from 
the cleavage stages so far described. The protospores enter 
on a period of growth and division, and since this growth 
occurs in connexion with the parent tissue and nourished by 
it, it may be fairly regarded as an embryonic stage. The first 
step in the further development of the protospores is a period 
of nuclear division, wherein the nuclei divide rapidly, so that 
the masses become once more multinucleated (Fig. 18-19). 
After the nuclei have thus divided the cell divides by con- 
striction, the nuclei being separated into two groups in the 
halves so formed. The nuclei may then divide still further, 
their divisions being followed by further cell-divisions, 
but finally nuclear division ceases. Cell-division, however, 
continues until the masses are cut up into regularly oblong 
binucleate cells. A series of these final cell-divisions, whereby 
an octinucleate mass is cut up into binucleate spores, is shown 
in figures 20 to 23. After each cell-division the nuclei are 
separated into two equal groups in opposite halves of the 
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